Even after Kudoa septempunctata parasitic in Olive flounder was found as a cause of food poisoning, similar cases of unknown cause were still taking place. Juvenile Pacific bluefin tuna (JPBT, hereafter) is often presented in those cases and it is suspected that K. hexapunctata parasitic in JPBT would be pathogenic for humans. We attempted to detect K. hexapunctata from JPBT leftover and patients' feces in 4 cases in Niigata prefecture from 2011 to 2014. In addition, we also investigated excretion of K. hexapunctata into the feces after intake of JPBT on a trial basis. The 4 cases occurred during summer from July to September. K. hexapunctata was detected from JPBT leftover and patients' feces; therefore, it was estimated that K. hexapunctata could be parasitic in JPBT eaten in all those cases. Furthermore, we also found that JPBT and Olive flounder were eaten together in 2 cases, but K. septempunctata was not detected from patients' feces. The detection rate of K. hexapunctata from patients' feces was 86% for feces collected within 3 days after meal, but it was lowered 25% for feces collected after 1 week. The quantity of K. hexapunctata excreted into feces after intake of JPBT on a trial basis tended to be declining with time, and then K. hexapunctata was detected in feces up to approximately 3 days after intake of JPBT. From those results, a fecal examination would be important as collecting feces after meal or during an early stage, and feces collected within 3 days after meal could be appropriate for the examination. Cases in relation to JPBT are similar to those of food poisoning caused by K. septempunctata and it is suspected that K. hexapunctata might be the cause. For K. hexapunctata, it would be necessary to accumulate and examine the case data while clarifying its pathogenicity.